A synthetic paper which is produced from a synthetic resin as a main starting material and which is a substitute for paper is superior in water resistance, dimensional stability upon moisture absorption, surface stability, gloss and vividness of printed images and mechanical strength. Based on these advantageous properties, the utility of the synthetic paper has been widening in recent years.
The main starting materials of the synthetic paper are polyolefin resins such as polyethylene and polypropylene, and polyester resins. Of these, polyester resins represented by polyethylene terephthalate are superior in that they have high heat resistance and nerve, and can be used for a wide range of applications.
As a method for preparing, from the main starting material of polyester resins, a film having functions similar to those of paper there have been conventionally known (1) a method wherein fine voids are formed inside the film in large amounts, (2) a method wherein general flat film surfaces are made rough by (2-1) sand blast treatment, (2-2) etching treatment, (2-3) matting treatment (a matting agent is laminated together with a binder) and the like, and other methods. The method (1) is advantageous in that the film itself can have reduced weights, and sufficient flexibility, fine writability and clear printing-transcription enabling properties can be imparted.
A conventionally known method for forming fine voids inside the film comprises melt-kneading a polyester resin and a thermoplastic resin incompatible therewith, preparing a sheet therefrom, with said thermoplastic resin dispersed in a fine particulate state in the polyester resin, and drawing said sheet, in particular, biaxially stretching same in the lengthwise direction and crosswise direction to produce voids around the fine particles.
The thermoplastic resin incompatible with the polyester resin, which is used for producing voids includes, for example, polyolefin resins (e.g. Japanese Patent Unexamined Publication No. 132755/1974), polystyrene resins (e.g. Japanese Patent Examined Publication Nos. 2016/1974, 29550/1979), and polyarylate resins (e.g. Japanese Patent Examined Publication No. 28097/1983). Of these, polypropylene and polystyrene are particularly preferable in that the voids are easily formed, they have low density and they are economically advantageous.
On the other hand, the method comprising biaxial stretching in the lengthwise direction and crosswise direction necessitates addition, in large amounts, of a resin incompatible with a polyester, to the polyester, which leads to substantial inhibition of biaxial stretching to the extent that the draw ratio (particularly lengthwise draw ratio) needs to be set considerably lower than in the case where a conventional polyester film is prepared.
Hence, most of the polyester films having fine voids which have been practically used as synthetic paper, etc. are subject to reinforced crosswise orientation, and have, aided by the voids formed inside, a tendency to allow crosswise tearing during processing wherein a tension is applied in the lengthwise direction of the film, thus posing problems in that the film easily breaks due to a slight flaw made in the film, and fins are easily produced on cutting.
In an attempt to overcome these defects, a film may be improved in isotropy by increasing the lengthwise draw ratio, or the lengthwise drawing may be reinforced. Such drawing is, however, very difficult in view of the above-mentioned inherent properties of the polyester resin having voids, and industrially impractical.
It may well be considered that, by reducing the crosswise draw ratio, the above-mentioned defects can be overcome using conventional techniques. For a crosswise drawing of a film to be uniformly done, a certain level of draw ratio is necessary. Alternatively, an attempt to overcome the defects in the conventional methods inevitably results in inconsistent properties, such as thickness of the film, and stable industrial production is impossible.
The above-mentioned biaxial stretching in the lengthwise direction and crosswise direction is most generally done by a method comprising roll drawing a continuous sheet of a polymer mixture in the longitudinal direction and tenter drawing same in the width direction. For example, the roll drawing (lengthwise drawing) is conducted, to form a multitude of voids, at a temperature of from 80.degree. C. to 100.degree. C. and draw ratio of 2.0-5.0, tenter drawing (crosswise direction) at a temperature of from 80.degree. C. to 140.degree. C. and draw ratio of 2.8-5.0, and then heat treatment after drawing at not less than 150.degree. C. (Japanese Patent Unexamined Publication Nos. 168441/1988, 193938/1988, 80247/1990, 284929/1990, 114817/1991 and 202540/1992).
The thermoplastic resin films having fine voids, which are obtained by conventional techniques, have defects in that curling by heat and wrinkles by heat tend to occur when used as heat-sensitive recording materials and printing sheets.
The present inventors have found that the voids in the thermoplastic resin films having fine voids, which are obtained by conventional technique, tend to become longer in the crosswise direction rather than the lengthwise direction and that this tendency is one of the factors which induces occurrence of curling by heat, and studied from various aspects of isotropy of the voids. By setting the temperature of crosswise drawing to not less than 140.degree. C., the crosswise growth of the voids was suppressed. Concurrently, however, the content of the void was insufficient and variations in film thickness became greater, making the film substantially unsuitable for industrial production. An attempt was made to increase lengthwise draw ratio to form lengthwisely long voids and subject the voids to crosswise drawing, thereby to ultimately make the voids isotropic. As a result, film forming property became poor, so that the film was unsuitable for industrial production. Also, the crosswise draw ratio was tentatively made smaller to result in greater variations in film thickness.
A first object of the present invention is to resolve the above-mentioned problems and provide a polyester film having fine voids, which has high isotropy and superior tear resistance, and a method for manufacture thereof.
A second object of the present invention is to resolve the above-mentioned problems and provide a thermoplastic resin film having fine voids, which has high isotropy, permits only less occurrence of curling by heat and has less variation in thickness, and a method for manufacture thereof.